Low Leakage High Performance Static Random Access Memory Cell Using Dual-Technology Transistors

ABSTRACT

A memory cell includes a storage element, a write circuit coupled to the storage element and a read circuit coupled to the storage element. At least a portion of the storage element and at least a portion of the write circuit are fabricated using a thicker functional gate oxide and at least a portion of the read circuit is fabricated using a thinner functional gate oxide.

CLAIM OF PRIORITY UNDER 35 U.S.C. 120

This application is a continuation of prior application Ser. No.12/357,938 entitled “LOW LEAKAGE HIGH PERFORMANCE STATIC RANDOM ACCESSMEMORY CELL USING DUAL-TECHNOLOGY TRANSISTORS,” filed 22 Jan. 2009,assigned to the assignee hereof and expressly incorporated by referenceherein.

FIELD OF DISCLOSURE

This disclosure relates generally to Static Random Access Memory (SRAM)cell designs and specifically to a design that uses dual-technologytransistors to achieve improved performance and power characteristics.

BACKGROUND

Static Random Access Memory (SRAM) cells are the basic building blocksof many memories. An exemplary conventional 6-transistor (6T) SRAM cellas illustrated in FIG. 1 comprises two cross-coupled inverters, eachinverter comprising a serially-connected P-channel Field EffectTransistor (PFET) and N-channel Field Effect Transistor (NFET), whichallows the 6T SRAM cell to store one bit of data. The 6T SRAM cell alsocomprises two NFET pass-gate transistors which allow reading data fromand writing data into the 6T SRAM cell. A conventional memory circuitmay incorporate multiple individual 6T SRAM cells.

Memories using 6T SRAM cells are commonly used as cache memories inmicroprocessors, digital signal processors (DSPs) and other integratedcircuits. As semiconductor processes scale to smaller and smallerminimum feature sizes, the performance of the 6T SRAM cell does notalways improve as much as the performance of the integrated circuitsthat rely on memories that employ the 6T SRAM cell. It is thereforedesirable to increase performance of the SRAM cells. One conventionaltechnique used to increase performance is to replace the 6T SRAM cellwith an 8-transistor (8T) SRAM cell as illustrated in FIG. 2. The 8TSRAM cell illustrated in FIG. 2 provides separate read and write pathsfor the bit of data stored in the 8T SRAM cell. The 8T SRAM cellincreases performance at the cost of increased leakage power due to thetwo additional transistors.

It is also desirable to reduce leakage power in order to reduce theoverall energy usage of an integrated circuit. Since cache memoriescommonly can represent a significant portion of an entire integratedcircuit and SRAM cells can represent a large portion of a cache memory,it is especially desirable to be able to reduce leakage power of theSRAM cells and consequently of the integrated circuit as a whole.

It is therefore desirable to develop techniques that increaseperformance and reduce leakage in SRAM cells.

SUMMARY OF THE DISCLOSURE

In a first embodiment of the invention, a memory cell comprises astorage element, a write circuit coupled to the storage element and aread circuit coupled to the storage element. At least a portion of thestorage element and at least a portion of the write circuit arefabricated using a thicker functional gate oxide and at least a portionof the read circuit is fabricated using a thinner functional gate oxide.

In a second embodiment of the invention, a memory cell comprises firstand second NFETs and first and second PFETs. The source terminals of thefirst and second NFETs are coupled to a ground potential and the sourceterminals of the first and second PFETs are coupled to a firstpotential. The gate terminal of the first NFET is coupled to the gateterminal of the first PFET and the gate terminal of the second NFET iscoupled to the gate terminal of the second PFET. The drain terminal ofthe first NFET is coupled to the drain terminal of the first PFET andthe drain terminal of the second NFET is coupled to the drain terminalof the second PFET. The gate terminals of the first NFET and first PFETare coupled to the drain terminals of the second NFET and second PFETand the gate terminals of the second NFET and second PFET are coupled tothe drain terminals of the first NFET and the first PFET. The memorycell further includes third and fourth NFETs. The gate terminals of thethird and fourth NFETs are coupled together and are adapted to becoupled to a write word line. The source terminals of the third andfourth NFETs each are adapted to be coupled to one of a pair ofcomplementary write bit lines. The drain terminal of the third NFET iscoupled to the gate terminals of the first NFET and first PFET, and thedrain terminal of the fourth NFET is coupled to the gate terminals ofthe second NFET and second PFET. The memory cell further includes fifthand sixth NFETs. The source terminal of the fifth NFET is coupled to aground potential. The gate terminal of the fifth NFET is coupled to thedrain terminals of the first NFET and first PFET. The drain terminal ofthe fifth NFET is coupled to the source terminal of the sixth NFET. Thegate terminal of the sixth NFET is adapted to be coupled to a read wordline. The drain terminal of the sixth NFET is adapted to be coupled to aread bit line.

In a third embodiment of the invention, a memory array comprises aplurality of memory cells. At least one of the plurality of memory cellsincludes a storage element, a write circuit coupled to the storageelement and a read circuit coupled to the storage element. At least aportion of the storage element and at least a portion of the writecircuit are fabricated using a thicker functional gate oxide and atleast a portion of the read circuit is fabricated using a thinnerfunctional gate oxide.

One advantage provided by embodiments of the teachings herein isincreased memory performance due to the use of higher performancetransistors in the read path of a memory cell. Another advantage isreduced leakage power of the memory cell due to the use of lower leakagetransistors in portions of the memory cell where higher performance isnot as beneficial.

It is understood that other embodiments of the teachings herein willbecome apparent to those skilled in the art from the following detaileddescription, wherein various embodiments of the teachings areillustrated. As will be realized, the teachings herein are capable ofother and different embodiments without departing from the spirit andscope of the teachings. Accordingly, the drawings and detaileddescription are to be regarded as illustrative in nature and not aslimiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the teachings of the present disclosure areillustrated by way of example, and not by way of limitation, in theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram of a conventional 6T SRAM cell;

FIG. 2 is a schematic diagram of a conventional 8T SRAM cell;

FIG. 3 is a schematic diagram of an 8T SRAM cell according to theteachings of the present disclosure;

FIG. 4 is a block diagram of a memory array incorporating the teachingsof the present disclosure; and

FIG. 5 is a block diagram showing an exemplary wireless communicationsystem in which an embodiment of the disclosure may be advantageouslyemployed.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of various exemplaryembodiments of the teachings of the present disclosure and is notintended to represent the only embodiments in which such teachings maybe practiced. The detailed description includes specific details for thepurpose of providing a thorough understanding of the teachings by way ofillustration and not limitation. It will be apparent to those skilled inthe art that the teachings of the present disclosure may be practiced ina variety of ways. In some instances, well known structures andcomponents are shown in block diagram form in order to avoid obscuringthe concepts of the present disclosure.

FIG. 3 is a schematic diagram of an 8-Transistor Static Random AccessMemory (8T SRAM) cell 300 according to the teachings of the presentdisclosure. The 8T SRAM cell 300 includes a storage element 302, a writecircuit 304 and a read circuit 306. Although the 8T SRAM cell 300illustrated in FIG. 3 has a single storage element 302, write circuit304 and read circuit 306, those skilled in the art will recognize thatother configurations employing multiple storage elements, write circuitsand read circuits may advantageously employ the teachings of the presentdisclosure.

In one embodiment, a triple gate oxide (TGO) manufacturing process isused in the fabrication of the 8T SRAM cell 300. The TGO manufacturingprocess provides three transistor gate oxide thicknesses with varyingperformance characteristics on a monolithic integrated circuit die. AnI/O gate oxide, which is commonly the thickest gate oxide available on agiven integrated circuit die, is used for transistors in circuits forinput to and output from the integrated circuit die (I/O devices). Twofunctional gate oxides, which are both commonly thinner than the I/Ogate oxide, are commonly used for the remainder of the devices on theintegrated circuit die (functional devices). The thicker functional gateoxide provides lower leakage at the cost of reduced performance. Thethinner functional gate oxide provides increased performance at the costof higher leakage.

While the 8T SRAM cell 300 is operating, at least a portion of thedevices in the storage element 302 will be turned on and will thus becontinuously subject to leakage. In one embodiment, the devices in thestorage element 302 may be fabricated using the thicker functional gateoxide. This results in lower leakage in the storage element 302, therebyreducing power consumption of the 8T SRAM cell 300. Because at least aportion of the devices in the storage element 302 are in continuousoperation, a reduction in leakage power as compared to other functionalgate oxide thicknesses may be realized.

In one embodiment, the storage element 302 includes cross-coupledinverters, shown here as PFETs 350 and 352 of the thicker functionalgate oxide coupled to NFETs 354 and 356 of the thicker functional gateoxide, coupled between a ground potential 340 and a first potential 342.This allows a single bit of information and its complement to be storedat nodes 358 and 360. Although the present embodiment is directedtowards cross-coupled inverters, those skilled in the art will recognizethat the teachings of the present disclosure also apply to other methodsof statically storing information.

The write circuit 304 may not need to be capable of high speed operationand thus it is not as beneficial to fabricate the devices in the writecircuit 304 using the thinner functional gate oxide. In one embodiment,at least a portion of the devices in write circuit 304 are fabricatedusing the thicker functional gate oxide. Accordingly, leakage powerconsumed by the write circuit 304 is reduced, thereby reducing powerconsumption of the 8T SRAM cell 300. However, at low supply voltages,the use of thicker functional gate oxide for at least a portion of thedevices in the write circuit 304 may lead to unacceptably low writeperformance and degraded write stability. In order to improve theperformance and stability of the write circuit 304 in such designs, awrite word line 324 may be adapted to be driven by a driver circuit 380.The driver circuit 380 is coupled to a second potential 390 that ishigher than the first potential 342, and is also coupled to the groundpotential 340.

In one embodiment, the write circuit 304 includes NFETs 326 and 328which are fabricated using the thicker functional gate oxide. The writecircuit 304 further includes the write word line 324 adapted to controlNFETs 326 and 328 such that values on complementary write bit lines 320and 322 may be written into the nodes 358 and 360. Although in thisembodiment an NFET pass-gate write circuit configuration has beendescribed, those skilled in the art will recognize that other types ofwrite circuits may be employed without departing from the scope of theteachings of the present disclosure.

The read circuit 306 includes NFETs 334 and 336, which are coupled tothe storage element 302 in order to allow the bit of data stored in thestorage element 302 to be read. The read circuit 306 further includes aread word line 332 adapted to control the NFET 334, and a read bit line330 coupled to the NFET 334. The NFET 336 is coupled between the NFET334 and a ground potential 340 and is controlled by the node 360. Theread word line 332 and read bit line 330 may be selectively controlledto allow the logical complement of a logical value stored at the node360 to be present on the read bit line 330. Those skilled in the artwill recognize that other read circuit configurations may be employedwithout departing from the scope of the teachings of the presentdisclosure.

In one embodiment, NFETs 334 and 336 in the read circuit 306 arefabricated using the thinner functional gate oxide. This results inincreased performance for read operations from the 8T SRAM cell 300,which is advantageous because read operations are commonly a limitingfactor in memory performance. However, the use of the thinner functionalgate oxide in the read circuit 306 may lead to increased leakage. It maybe advantageous to use other techniques in order to reduce leakage inthe read circuit 306. For example, a source bias may be applied to atleast a portion of the read circuit to turn off devices in the readcircuit 306 more effectively. Also, a footer device (not shown) may beadded to the read circuit 306 to allow the remainder of the read circuit306 to be disconnected from the ground potential 340.

In an exemplary 45 nm TGO process, the first potential is 0.9 volts andthe second potential is 1.1 volts. However, other voltages may be usedwithout departing from the teachings of the present disclosure.Additionally, both the first potential 342 and the second potential 390may be adapted to be variable voltage supplies. For example, the firstpotential 342 may vary between 0.6 volts and 0.9 volts depending on amode of operation of a device incorporating the 8T SRAM cell 300.

In the present embodiment, all of the devices in the storage element 302and the write circuit 304 are of the thicker functional gate oxide andall of the devices in the read circuit 306 are of the thinner functionalgate oxide. However, those skilled in the art will recognize that it ispossible to realize some benefit in power consumption or performanceeven if not all of the devices in the storage element 302 and the writecircuit 304 are fabricated using the thicker functional gate oxide andnot all of the devices in the read circuit 306 are fabricated using thethinner functional gate oxide. For example, in applications where it isparticularly advantageous to reduce power consumption during readoperations, the NFET 336 of the read circuit 306 could be fabricatedusing the thicker functional gate oxide to reduce power since the gateof the NFET 336 is coupled directly to the storage element and thus,frequent switching of the NFET 336 may be unlikely. In such anapplication, the NFET 334 could be fabricated using the thinnerfunctional gate oxide to retain some performance benefit.

FIG. 4 is a block diagram of a memory array 400 incorporating theteachings of the present disclosure. The memory array includes a memorycell array 402 with m rows and n columns of exemplary 8T SRAM cells asshown in FIG. 3 that incorporate the teachings of the present disclosureas previously described. 8T SRAM cells 450, 460 and 470 represent the(1,n), (m,n) and (m,1) cells of the array, respectively. Each rowincludes a write word line driver 410 coupled to a write word line 412and a read word line driver 420 coupled to a read word line 422. Boththe write word line 412 and the read word line 422 are coupled to eachof the memory cells comprising the row; for example, cells 450 through460 which represent memory cells (1,n) through (m,n). Each columnincludes a write bit line 430 and a read bit line 440. Both the writebit line 430 and read bit line 440 are coupled to each of the memorycells comprising the row; for example, cells 460 through 470 whichrepresent memory cells (m,n) through (m,1).

Examples of memory array structures where the teachings of the presentdisclosure may be advantageously employed include but are not limited tocache memories or off-chip memories. Such cache memories or off-chipmemories may be incorporated into consumer electronic devices such ascellular phones, portable digital assistants (PDAs) or laptop computers.

When the leakage of an 8T SRAM cell 300 according to the presentdisclosure is compared to that of an 8T SRAM cell as previously known inthe art (and assuming that the individual devices in each SRAM cell havethe same dimensions), a significant reduction of leakage may beachieved. In simulations, a reduction of leakage in the range of 95-98%was observed. However, the use of thicker gate oxide devices in the 8TSRAM cell 300 of the present disclosure may result in reducedperformance of the cell. In order to realize both the power savingsprovided by the teachings of this disclosure and an acceptable level ofperformance in a larger memory structure, the individual devices of the8T SRAM cell 300 can be larger than those in the SRAM cells alreadyknown in the art. Conventionally, this is not a preferred approachbecause as device sizes increase, the size of each individual 8T SRAMcell 300 increases and leads to increased chip area and increased powerconsumption. However, even after the individual devices of the 8T SRAMcell 300 have been re-sized to achieve the desired performance goals, asignificant reduction of leakage power may still be realized by makinguse of the teachings of the present disclosure. In simulations takingre-sizing as described above into account, a reduction of leakage powerin the range of 50-75% was observed.

The teachings of the present disclosure may be advantageously combinedwith other techniques for reducing power. For example, both the read andwrite bit lines may be disconnected or allowed to “float” while they arenot being actively used.

FIG. 5 is a block diagram showing an exemplary wireless communicationsystem 500 in which an embodiment of the disclosure may beadvantageously employed. For purposes of illustration, FIG. 5 showsthree remote units 520, 530, and 550 and two base stations 540. It willbe recognized that typical wireless communication systems may have manymore remote units and base stations. Remote units 520, 530, and 550include IC devices 525A, 525B and 525C that include the circuitrydisclosed here. It will be recognized that any device containing an ICmay also include the circuitry disclosed here, including the basestations, switching devices, and network equipment. FIG. 5 shows forwardlink signals 580 from the base stations 540 to the remote units 520,530, and 550 and reverse link signals 590 from the remote units 520,530, and 550 to base stations 540.

In FIG. 5, remote unit 520 is shown as a mobile telephone, remote unit530 is shown as a portable computer, and remote unit 550 is shown as afixed location remote unit in a wireless local loop system. For example,the remote units may be cell phones, hand-held personal communicationsystems (PCS) units, portable data units such as personal dataassistants, or fixed location data units such as meter readingequipment. Although FIG. 5 illustrates remote units according to theteachings of the disclosure, the disclosure is not limited to theseexemplary illustrated units. The disclosure may be suitably employed inany device which includes integrated circuits.

While the teachings of the present disclosure are disclosed in thecontext of SRAM cells, it will be recognized that a wide variety ofimplementations may be employed by persons of ordinary skill in the artconsistent with the teachings herein and the claims which follow below.

1. A memory cell, comprising: a storage element; a write circuit coupledto the storage element; and a read circuit coupled to the storageelement; wherein at least a portion of the storage element and at leasta portion of the write circuit are fabricated using a thicker functionalgate oxide and at least a portion of the read circuit is fabricatedusing a thinner functional gate oxide.
 2. The memory cell of claim 1,wherein the write circuit comprises a write word line and wherein thewrite word line is adapted to be coupled to a first potential, and theremainder of the write circuit, the read circuit and the storage elementare adapted to be coupled to a second potential that differs from thefirst potential.
 3. The memory cell of claim 2, wherein the firstpotential is adapted to be varied.
 4. The memory cell of claim 2,wherein the second potential is adapted to be varied.
 5. The memory cellof claim 1, wherein the thicker functional gate oxide provides lowerleakage than the thinner functional gate oxide.
 6. The memory cell ofclaim 1, wherein the thinner functional gate oxide provides higherperformance than the thicker functional gate oxide.
 7. The write circuitof claim 1, further comprising a write bit line adapted to float when nowrite operation is being performed.
 8. The memory cell of claim 1,wherein the read circuit comprises a read bit line.
 9. The memory cellof claim 8, wherein the read bit line is adapted to float when no readoperation is being performed.
 10. The memory cell of claim 1, whereinthe read circuit further comprises a footer device.
 11. The memory cellof claim 10, wherein the footer device is coupled between a groundpotential and a remainder of the read circuit.
 12. The memory cell ofclaim 1, wherein a source bias is applied to at least a portion of theread circuit.
 13. The memory cell of claim 1 further comprising aplurality of write circuits coupled to the storage element.
 14. Thememory cell of claim 1 further comprising a plurality of read circuitscoupled to the storage element.
 15. The memory cell of claim 1, whereinthe memory cell is disposed in one of the group consisting of: amicroprocessor, a digital signal processor, and a memory array.
 16. Amemory cell, comprising: a storage element comprising first and secondNFETs and first and second PFETs, wherein the source terminals of eachNFET are coupled to a ground potential and the source terminals of eachPFET are coupled to a first potential, wherein the gate terminal of thefirst NFET is coupled to the gate terminal of the first PFET, whereinthe gate terminal of the second NFET is coupled to the gate terminal ofthe second PFET, wherein the drain terminal of the first NFET is coupledto the drain terminal of the first PFET, wherein the drain terminal ofthe second NFET is coupled to the drain terminal of the second PFET, andwherein the gate terminals of the first NFET and first PFET are coupledto the drain terminals of the second NFET and second PFET and the gateterminals of the second NFET and second PFET are coupled to the drainterminals of the first NFET and the first PFET; a write circuitcomprising third and fourth NFETs, wherein the gate terminals of thethird and fourth NFETs are coupled together and are adapted to becoupled to a write word line, wherein the source terminals of the thirdand fourth NFETs are each adapted to be coupled to one of a pair ofcomplementary bit lines, wherein the drain terminal of the third NFET iscoupled to the gate terminals of the first NFET and first PFET, andwherein the drain terminal of the fourth NFET is coupled to the gateterminals of the second NFET and second PFET; and a read circuitcomprising fifth and sixth NFETs, wherein the source terminal of thefifth NFET is coupled to the ground potential, wherein the gate terminalof the fifth NFET is coupled to the drain terminals of the first NFETand first PFET, wherein the drain terminal of the fifth NFET is coupledto the source terminal of the sixth NFET, wherein the gate terminal ofthe sixth NFET is adapted to be coupled to a read word line, and whereinthe drain terminal of the sixth NFET is adapted to be coupled to a readbit line.
 17. A memory cell, comprising: means for storing data; meansfor writing data into the means for storing data, the means for writingdata coupled to the means for storing data; and means for reading datafrom the means for storing data, the means for reading data beingcoupled to the means for storing data; wherein at least a portion of themeans for storing data and at least a portion of the means for writingdata are fabricated using a thicker functional gate oxide and at least aportion of the means for reading data is fabricated using a thinnerfunctional gate oxide.
 18. A memory array comprising a plurality ofmemory cells, wherein at least one memory cell comprises: a storageelement; a write circuit coupled to the storage element; and a readcircuit coupled to the storage element; wherein at least a portion ofthe storage element and at least a portion of the write circuit arefabricated using a thicker functional gate oxide and at least a portionof the read circuit is fabricated using a thinner functional gate oxide.19. The memory array of claim 18 wherein the write circuit furthercomprises a write word line, wherein the write word line is adapted tobe coupled to a first potential, and the remainder of the write circuit,the read circuit and the storage element are adapted to be coupled to asecond potential.
 20. The memory array of claim 18, wherein the memoryarray is disposed in one of the group consisting of: a microprocessor, adigital signal processor, and a memory.